Various types of optical sensor make use of interferometers to detect changes in a physical parameter. In such sensors, light from a laser is coupled into an interferometer, which is influenced by changes in the physical parameter to produce corresponding changes in the interference pattern. These changes in the interference pattern manifest as changes in intensity, which can be detected by a photodetector.
Various different physical parameters can be used to cause a change in the interference pattern and hence can be sensed by this type of sensor. Examples include pressure (including air pressure), strain and displacement.
The signal-to-noise ratio (SNR) of such sensors is often limited by noise caused by fluctuations in intensity or frequency of the light output from the laser. There are various methods for stabilizing laser frequency. For example, one method makes use of a Fabry-Pérot interferometer or etalon to generate an error signal. The etalon converts frequency fluctuations into intensity fluctuations, which can be detected by a photodetector. The resulting photocurrent is used as a feedback signal, which can either act on the laser supply current or move a cavity mirror, for example, to correct the frequency fluctuations.
However, this kind of arrangement is bulky and expensive. It is incompatible with many applications where use of optical sensors would be desirable. For example, in mobile communication devices, microphones are required that are highly stable and resistant to shock, insusceptible to wind noise. These features can all be provided by an optical microphone making use of a laser light source and interferometer since these do not have moving parts, such as a membrane. However, it is also required that such microphones are compact and have a high SNR. Whilst the stabilization technique referred to above can compensate for laser frequency noise to a certain extent, it cannot compensate of relative intensity noise.